I'm not so much interested in hearing how "Enterprise" screwed it up. What I'm interested is how you would create a show about the early days of Earth's contact with new life and new civilizations.
Think about it from the point of view of someone trying to design a TV series. Remember, if you use fan-based designs (such as the Starfleet Museum, the Franz Joseph tech manuuals, etc.), you will using someone else's intellectual property.
All you have to start from is TOS, and any elements from the later series which could logically have existed in a prequel to those series. What known aliens could be included? Would you have any alien crew members?
How would you make the technology look more "primitive" than the NCC-1701? Would transporters and deflectors exist? What kind of weaponry would you include?
What themes would you like to explore in such a series? What are some of the stories would you be able to tell (how the Prime Directive came about? Who did humans first come in contact with?)
These are just some questions and ideas to get the job rolling.